The subject matter of this specification relates generally to storing and restoring digital data.
Modern graphical user interfaces allow a large number of graphical objects or items to be displayed on a display screen at the same time. Operating systems, e.g., Apple Mac OS®, provide user interfaces in which a number of graphical user interface windows can be displayed, overlapped, resized, moved, configured, and reformatted according to the needs of a user or a particular application. Taskbars, menus, virtual buttons, and other user interface elements provide mechanisms for accessing and activating windows even when they are hidden behind other windows.
With the sophisticated tools available, users are encouraged not only to create and save a multitude of items in their computers, but to revise or otherwise improve on them over time. For example, a user can work with a certain file and thereafter save the file's current version on a storage device. The next day, however, the user could have had second thoughts about the revisions, or could have come up with new ideas, and therefore opens the file again.
The revision process is usually straightforward if the user wants to add more material to the file or make changes to what is there. But it is typically more difficult for a user who has changed his/her mind about changes that were previously made and wants the file returned to a previous version. Application programs for word processing typically let the user “undo” previous edits of a text, at least up to a predefined number of past revisions. The undo feature also usually is configured so that the previously made revisions must be undone in reverse chronological order; that is, the user must first undo the most recently made edit, then the second-most recent one, and so on. If the user saves and closes the document and thereafter opens the document again, it might not be possible to automatically undo any previous edits.